This section provides background information related to the present disclosure which is not necessarily prior art.
Information display technologies are being developed from achieving high performance and functionality for showing objects with reality toward providing mobility and maximizing convenience, along with development of mobile terminals, such as mobile phones, PDA, etc. In order to meet recent requirements for information display, demands for flexible displays (FDs) that are free from spatial and structural limits while having a light weight and being easily folded are greatly increasing.
A flexible display is a display fabricated with a thin, flexible substrate that can be curved or bent. The flexible display can be classified into a rugged display, a bendable display, and a rollable display. The flexible display is continuously being studied and developed with the aim of commercializing a paper-like display, in order to implement a variety of applications without spatial and structural limits by substituting heavy, fragile sheet glass having been used in conventional Flat Panel Display (FPD), such as a TFT substrate for LCD or organic EL display, a color filter substrate, a substrate for touch screen panel, a substrate for solar cell, etc., for a thin, flexible substrate.
Main technologies for implementing such a flexible display include technologies of discovering materials for flexible substrate, of discovering organic and inorganic materials suitable for a low-temperature process, and of sealing flexible electronics. The technology of developing materials for flexible substrate among the above-mentioned technologies is considered as an essential technology since a substrate material has a great influence on deciding the performance, reliability, and price of a display.
Essential properties that are required to materials for flexible substrate are thinness, light-weight, low cost, and process fit. As substrate materials that satisfy the properties, metal foil, very thin glass, polymer plastic, etc. have been put into consideration. For industrial commercialization, it is essential to ensure yield through roll-to-roll process, and in yield terms, a polymer plastic substrate is receiving an increasing amount of attention.
Metal foil needs introduction of separate insulated coating layers although it has thermal resistance and flexibility, and also is naturally limited in technical fields requiring transparency. Very thin glass has advantages of good surface flatness and low water and oxygen permeability, while having disadvantages of weakness against impact and insuitability for roll-to-roll process compared to other materials for substrate. Meanwhile, polymer plastic is attracted as the most prospective substrate material since it does not have the above-mentioned disadvantages, is light-weighted, and also easy to be processed without structural limits.
Polymer for fabricating such polymer plastic substrates includes polyethylene terephthalate (PET), polyethersulfone (PES), polyethylene naphthalate (PEN), polyacrylate (PAR), polycarbonate (PC), polyimide (PI), etc. However, conventional plastic substrates have a critical disadvantage of having a great coefficient of thermal expansion (CTE). In other words, generally, polymer physically changes by segmental motion of local molecular chains near a glass transition temperature Tg of polymer, so that the polymer becomes subject to a sharp change in dimension in a high temperature process above the glass transition temperature Tg. Generally, glass has a CTE of several ppm/, while polymer has a relatively great CTE reaching several tens of ppm/° C. The great CTE of polymer may cause problems of deteriorating dimensional stability upon fabricating devices such as TFT on a plastic substrate and generating crack and peel-off of inorganic layers. Recently, studies into composite resins using thermosetting epoxy resin or transparent acrylic resin to which glass filter is added in order to overcome the disadvantages of the plastic substrate material are underway. Such composite resins are disclosed in Japanese Laid-open Patent Applications No. 06-337408, No. 2001-59015, No. 54-24993, No. 06-094523 and No. 05-140376. In the case where thermosetting epoxy resin is used in substrates formed with conventional composite resins, impact resistance becomes low due to strong brittleness of epoxy resin, which makes it difficult to fabricate a flexible film. In the case of using transparent polymer resin with glass filler, birefringent may occur due to a difference in index of refraction. Also, there is still a disadvantage of a great CTE and insufficient thermal resistance so that high dimensional stability can no longer be maintained.
Recently, a resin composition which can overcome a problem of index of refraction when glass filler is added to the resin composition using unhydrolyzation reaction of siloxane, apart from the above-mentioned material for plastic substrate and its composition, has been introduced. For example, Korea Laid-open Patent Application No. 10-2010-0083697 discloses a composition having high transparency and a small CTE by using transparent siloxane resin created by unhydrolyzation reaction of glass filler. However, since the composition also still has a weak mechanical strength and low durability and particularly has low adhesiveness with respect to inorganic layers, there is the possibility that the composition will be cracked and peeled off during a fabrication process. Also, due to a batch process in which glass filler is impregnated and additional yield reaction has to be performed, there is limitation in fabricating films by a roll-to-roll process for low cost other than sheets, and particularly, there are difficulties in discovering a substrate substance requiring a thin thickness below 20 μm.
As a result of studies into various polymer materials that can be used as a plastic substrate material, available polymer and its composition, other than the polymer for plastic substrate as mentioned above, have been discovered.
Essential physical properties required to polymer as a material for plastic substrate are as follows.
1) Dimensional stability
2) Small CTE
3) High barrier property
4) Rigidity
5) High visible light transmittance
6) Durability
Other than the physical properties mentioned above, thinness, light-weight, and ensuring a roll-to-roll process are needed, and also compatibility capable of being applied to various types of display substrates is required.
First, dimensional stability is required to minimize deformation caused by expansion and contraction when a polymer substrate is exposed to a maximum process temperature and time. Thermal resistance becomes a barometer of dimensional stability. Generally, thermal resistance at a temperature of 200° C. or higher is required, and in some high temperature processes, thermal resistance at a temperature of 300° C. or higher is required.
In the case of a CTE, generally, a CTE of 20 ppm/° C. or lower is preferable; however, there is no polymer substrate that satisfies the CTE of 20 ppm/° C. An inorganic material (particularly, a driving part) is deposited on a plastic substrate, and if a difference in CTE between the inorganic material and the plastic substrate is great, the inorganic layer may be cracked or peeled off. Accordingly, a material having a small CTE is preferably used to fabricate a flexible display.
Then, in the case of barrier property, requirements for use as a substrate material that can substitute for glass are very critical. Oxygen, water, particles, etc. incoming from the outside may contaminate liquid crystal or driving devices or may oxidize electrode metal layers in the case of OLED, which may influence lifetime reduction of the display.
Generally, moisture vapor permeability of polymer has a great value of 1 to 100 g/m2[day], LCD requires moisture vapor permeability below 10−2 g/m2[day], and OLED requires moisture vapor permeability below 10−6 g/m2[day].
Then, rigidity has important meaning in determining process fit of substrates, and is defined by a function of young's modulus (E), thickness (t), and poisson's ratio (v). Rigidity can be expressed by Equation (1):Rigidity=E×t3/12(1−v)  (1)
A plastic substrate requires appropriate rigidity, and it is seen from equation 1 that rigidity is better as a substrate deformation rate is low and its elastic modulus is great regardless of changes in thickness.
Then, in the case of visible light transmittance, high visible light transmittance has to be maintained without scattering or changes in reflectance due to a degree of crystallization or a change in surface of a polymer material, which are important material properties having great influence on image characteristics and consumption power of a display. Most of the existing optical substrates show high transmittance exceeding 85% at the wavelength of 550 nm and have small differences in transmittance from films applied thereon.
Finally, durability has a great influence on a lifetime of a flexible plastic substrate. That is, since a plastic substrate has a multi-layer structure where organic and inorganic materials, such as a base substrate, a barrier, transparent electrodes, etc., are applied, internal stress is caused when the plastic substrate is bent or curved, and adhesive failure occurs on a thin film applied on the plastic substrate, accompanied by cohesive failure made in a thin film applied on the plastic substrate. Accordingly, in order to implement a paper-like display using a flexible substrate, durability has to be necessarily guaranteed.
In order to acquire the required physical properties, studies into materials for plastic substrates have been actively conducted, however, appropriate polymer that satisfies all the required physical properties has not yet been found out.
Among the above-mentioned polymer materials, the PC has excellent mechanical, optical properties but has low chemical resistance, so that it has limitation of available solvents (PR developer, PR remover, metal etchant cleaning solvent). In order to overcome the problem to ensure chemical resistance on both sides of a PC substrate, it is necessary to form a separate chemical resistance layer. Also, the PC substrate has resistance against ultraviolet light, which makes limitation upon substrate processing. Besides, the PC substrate contains much out-gas and its CTE is 10 times greater than inorganic materials. Furthermore, the PC substrate has to be processed at a lower temperature (a maximum temperature range of about 150 to about 180° C.) than existing glass substrates.
The PET has been examined as a material for substrate for a long time since the PET has low water absorption and a low melting temperature so that it can be used to fabricate polymer substrates at a relatively low temperature with low cost. However, the PET shows poor thermal stability and optical anisotropy due to a low Tg, and accordingly it cannot be applied to substrates for display that use a polarizer such as LCD. Particularly, PET molecule chains are recrystallized upon heat treatment above 150° C. to cause a whitening phenomenon in which the substrate becomes locally white, thereby degrading optical transmittance of the substrate.
Next, the PEN, which is plastic having optical anisotropy that acts similar to the PET described above, has the problem similar as in the PET in view of thermal stability. However, recently, studies into a possibility that the PEN can be used as a material for substrate are being conducted.
The PES, which was initially commercialized by Sumitomo Bakelite Co., Ltd. in Japan, has good thermal resistance, and accordingly, studies into using the PEC as a material for substrate are actively being conducted. However, the PES has poor chemical resistance, a great CTE, and a low maximum process temperature (e.g., about 180° C.), and requires an additional dehydration process when the PES as polymer having high hygroscopic property contacts water or is exposed to air for a long time. Furthermore, the PES is expensive compared to glass.
Next, the PI, which is a polymer film that is widely applied to electrical and electronic components due to its excellent chemical and thermal resistance, has more excellent hygroscopic property (about 3 times) than the PES and requires an appropriate dehydration process necessarily. An imide group of each molecular chain of PI offers thermal resistance and simultaneously becomes a factor of generating yellowing. Particularly, light transmission of the PI is about 30% to about 50% with respect of visible light having a wavelength of 550 nm, and such low optical property becomes a roadblock to application as a substrate material for display. Recently, Mitsubishi Gas Co., Ltd. was developed a PI substrate (a product name Neopulim™) that is optically transparent (visible light transmittance of 80% or higher) and has a high Tg (>300° C.) using changes in PI molecule chains.
Meanwhile, an AryLite substrate is an optical substrate that was developed by Ferrania Image System in Italia, and shows excellent thermal, optical and chemical characteristics compared to other plastic substrate materials. However, the AryLite substrate also has a great CTE and UV resistance, which make limitation upon substrate processing. Furthermore, AryLite is expensive compared to glass.
For these reasons, as a plastic substrate material for flexible display capable of substituting for a glass substrate, a new polymer composition for plastic substrate having more excellent physical properties than conventional plastic substrate materials is needed.